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286

ALLOYS OF GOLD AND COPPER

GOLD   AND   ITS   ALLOYS

The more important commercial products are : gold in bars, sheet,
granules, powder, etc., and its alloys with copper alone and with silver and
copper.

GOLD

The essential determination to be carried out on the metal is that of
the gold, the fineness being the amount of gold per 1000 parts of the metal.
This determination is made as in alloys of gold and copper (q.v.).

ALLOYS   OF   GOLD   AND   COPPER

The most important determination is :

1. Determination of the Gold.óBy cupellation in presence of lead,
the gold is separated from copper and other ordinary metals with which
it may be alloyed, but not from silver, which resembles gold in being un-
oxidisable at the highest temperatures. To eliminate the silver, which
always accompanies gold in larger or smaller proportion, it is necessary to
treat with acid. Experience has shown that, for the complete elimination
of the silver, the latter must be in considerable excess, namely, about 3
parts to i part of gold. It is, therefore, necessary, before cupellation to
add silver to make this relation hold. The assay of gold hence comprises
two distinct operations:

(1)  Cupellation in presence of lead and silver to eliminate the base
metals and to form the alloy of gold and silver in the above proportions,
an operation termed inquartation, since the gold constitutes about one-
fourth of the resulting alloy.

(2)  Treatment of the latter with acid to remove the silver, this operation
being known as parting.

To calculate the quantities of silver and lead to be used in the cupellation
the gold content of the sample must be known approximately. It is neces-
sary, therefore, to make a preliminary assay and this is usually done by
means of the touchstone.

PRELIMINARY TOUCHSTONE ASSAY. This consists in tracing a streak
with the sample on the touchstone beside streaks traced with gold-copper
alloys of known fineness and comparing the colours of the streaks before and
after treatment with acid.

This test requires:

(1)   The touchstone.   A good stone should be unattackable by acid and
should be of a uniform black colour, hard, of fine grain and opaque surface.

(2)  The needles, consisting of small, thick discs or plates of gold-copper
alloys of definite fineness, fixed to metallic handles on which the fineness is
marked.

(3)  The acids, which vary in composition according to the fineness of
the alloy to be tested.   The acids generally used are the so-called 500 acid acid (D 1-2).    In presence